1. Field of the Invention
The present invention relates to a multiple quantum well (MQW) semiconductor layer (hereinafter simply called MQW laser) whose threshold current has a small temperature dependency.
2. Description of the Related Art
Recently, MQW lasers whose threshold current has a small temperature dependency are available which use an MQW structure as an active layer and have a compressive and strained quantum well (QW) layer to reduce the two-dimensional density (hereinafter referred to as N.sup.2D.sub.th) per QW layer.
FIG. 1 shows the structure of a conventional MQW laser with an oscillation wavelength of 1.3 .mu.m, whose threshold current has a small temperature dependency, and FIG. 2 shows the structure of the MQW active layer of this MQW laser (19th European Conference on Optical Communication, Proceedings, Vol. 3, p. 21 by A. Oishi et al.). As shown in FIG. 1, the MQW laser comprises a p type InP substrate 1, a 1.5-.mu.m wide stripe structure 12, which has a p type InP clad layer 2, an MQW active layer 3 and an n type InP clad layer 4 formed on the substrate 1, and a current block structure 13, which has a p type InP layer 5, an n type InP current block layer 6 and a p type InP layer 7 formed on both sides of this stripe structure 12. An n type InP layer 8, an n type InGaAs contact layer 9 and an n electrode 10 are formed on the stripe structure 12 and the current block structure 13. A p electrode 11 is formed at the back of the substrate 1.
As shown in FIG. 2, the MQW active layer 3 has an MQW structure in which a 20-nm thick p type InGaAsP light guide layer 21 with a bandgap wavelength of 1.15 .mu.m and a 150-nm thick n type InGaAsP light guide layer 24 with a bandgap wavelength 1.15 .mu.m sandwich a 5-element MQW comprising 1.4%-compressively strained five 4-nm thick InGaAsP QW layers 22, 10-nm thick four InGaAsP barrier layers 23 with a bandgap wavelength of 1.15 .mu.m, located between the adjoining QW layers 22.
The conventional MQW laser has a temperature characteristic showing a threshold current of 1.8 mA at 25.degree. C. and a threshold current of 4.7 mA at 80.degree. C.
To suppress the temperature dependency of the threshold current more than the prior art, it is important to reduce the two-dimensional carrier density N.sup.2D.sub.th per QW layer for the following reason. The threshold current is temperature dependent mainly because the temperature dependency of the ratio of the Auger recombination is large. Reducing N.sup.2D.sub.th can lower the ratio of the Auger recombination. Theoretically, to reduce N.sup.2D.sub.th, it is effective to shorten the wavelength of the barrier layer composition (e.g., IEEE J. Quantum Electron. Vol. 29, p. 885, 1993 by M. Nido et al.).
In the MQW laser with the conventional structure as shown in FIG. 1, the p type InP clad layer 2 and the n type InP clad layer 4 for injecting the carrier into the MQW active layer 3 are not in direct contact with the individual QW layers 22. Although the p type InP layer 5 directs contacts the individual QW layers 22, the p type InP layer 5 has a thickness of about 1 .mu.m at a location sufficiently apart from the stripe structure 12 and is thinner than about 1 .mu.m on the sides of the stripe structure 12. Thus, the p type InP layer 5 has a high resistance. The stripe structure 12 has a greater width than the thickness of the MQW active layer 3. That is, the holes are injected into the QW layers 22 mainly from the p type InP clad layer 2, not from the p type InP layer 5. Accordingly, the carrier is diffused in the MQW structure while repeating the trapping and thermal excitation between the light guide layer 21 and barrier layers 22 and the QW layers 22, and is injected into each QW layer 22.
Since the ratio of the thermal excitation from the hole trapped state into the QW to the non-trapped state is small, the holes are locally present in some QW layers close to the p type InP clad layer 2 in the MQW active layer 3 when the carrier is injected. When the holes are locally present, electrons which have light effective mass tend to be locally present, due to the Coulomb force, in the vicinity of where the holes are locally located (e.g., IEEE J. Quantum Electron. Vol. 29, p. 1586, 1993 by N. Tessler et al.).
The local presence of the carrier is not so significant when the barrier layer composition has a long wavelength as in the case of the conventional laser shown in FIG. 1 (1.15 .mu.m in the case of FIG. 1), but it becomes more prominent when the wavelength of the barrier layer composition is shortened further. This is because when the wavelength of the barrier layer composition becomes shorter, the ratio of the thermal excitation from the hole trapped state into the QW to the non-trapped state becomes substantially the same as the ratio of the natural discharge caused by the reuniting of the electrons and holes in the QW layers 22. The rise in N.sup.2D.sub.th due to the local presence of the carrier is not negligible, so that actually, the shortening of the wavelength of the barrier layer composition does not always result in a reduction in the temperature dependency of the threshold current.
In other words, even when the wavelength of only the barrier layer composition in the MQW active layer 3 in the conventional structure as shown in FIG. 1 is shortened, the temperature dependency of the threshold current cannot be reduced.